Make the tests a bit easier to read.

Add comments and indentation.

Handle XL C a bit more like the other compilers.  Make the first test be
"is this someting that's not XL C", as is the case for other compilers;
that test is done as "are both __xlC__ and __ibmxl__ undefined".  If
either of them are defined, use __ibmxl_version__ if __ibmxl__ is
defined, and use __xlC__ otherwise.

(cherry picked from commit 1912902e96bcb1729d58225541c5e5b75345886a)